Multiclass Abominations have always been OP in any game.
Power balance issue on 3.5 and Pathfinder (not early) was really shock, and DM should organize them with giving poor PCs good items, or prohibit multi-class which is OP, these kind of issues are more terrible than 5e on my opinion, even bad for making game as Computer game.
I disagree here. I've DM'd a lot of 5e. Multiclasses can be strong, but they also lose out on basic stat and spell progression.
It's mainly skill access and some OK spell/ability combos that helps with Multiclass. And again, it encourages people to use different types of Races more.
A basic Moon Druid can wipe the floor with everyone in an early game of D&D 5e, without needing much of anything, but for every class like that there's a Dozen others which really benefit from a small 1-3 level Dip.